The present invention relates to luminous signalling devices, for instance luminous signalling panels of the type illustrated in FIG. 1 of the attached drawings. Panels of this type are contemplated in the safety standards currently in force for indicating, for example, emergency exits from premises.
According to traditional techniques, the luminous panels of the type illustrated in FIG. 1 have a hollow supporting body containing inside a light source, for example a phosphorescent lamp, for the backlighting of the face of the panel bearing the image that is to be displayed.
The purpose of the present invention is to provide a luminous signalling device which, as compared with known devices, has a far more compact and simple structure and a more efficient operation.
With a view to achieving the above purpose, the subject of the invention is a luminous signalling device comprising:
a body made of transparent synthetic material, basically in the form of a plate having two larger faces set opposite to one another, at least one of which bears an image, two end walls, and two longitudinal walls;
a plurality of LED devices, some of which designed to emit white light, and others designed to emit green light, externally coupled to at least one of said end walls or said longitudinal walls, the arrangement being such that the rays of light coming out of said LED devices undergo multiple total reflections within the plate, which thus functions as a light guide; and
a distribution of diffusing, diffractive optical elements formed on the face bearing the image in areas corresponding to selected areas of the image designed to diffuse outwards the rays of light that are incident on said areas from within the plate.
The above-mentioned plate body may, for example, be made of polymethyl methacrylate or polycarbonate. LEDs (light-emitting diodes) that emit white light have been developed in relatively recent years in a form usable at an industrial scale. Particularly suitable for the purposes of the invention are the models NSCW 100 and NSSW 440 manufactured by the Company NICHIA. These white-light emitting diodes typically have the structure illustrated in FIG. 3 of the attached drawings. The LED proper, designated by 1, is set at the bottom of a cavity 2 made in a support 3 and filled with an epoxy resin matrix 4 englobing multicolour phosphors. The LED 1 is designed to emit blue light with a short wavelength but, following on interaction with the phosphors in the resin 4, the light rays emitted by the device are white-light rays. As indicated above, according to the invention a plurality of white-LED devices of the surface-mounted device (SMD) type, that is, a type suited for being mounted on a surface, is arranged. The various LEDs associated to a wall of the plate body of the device may be supplied by means of a channel and may be rendered adherent to the plate by means of a polymer layer having the additional function of improving LED/plate coupling. The aforesaid diffusing optical elements arranged in areas corresponding to selected areas of the image carried on one or on both of the faces of the plate body may, for example, be diffractive lenses or micro-refractive lenses. These diffusing optical elements are made by moulding or pressing (for instance, by injection-moulding or by a hot-pressing process) on the face of the plate body.
In the typical case exemplified in FIG. 1 of the annexed drawings, in which the image to be displayed presents a lighter-coloured shape on a darker background, for instance a white shape on a green background, the aforesaid diffusing optical elements are formed in areas corresponding to the light-coloured shape. However, it is also possible to arrange optical elements also in areas corresponding to the dark background of the image, differentiating the optical elements from one another so as to control the respective luminances. The image may be obtained on the face of the plate body, for example by means of silkscreen printing.
The device according to the invention may be made like a flag, with the image on both of the larger faces, or else also with the image on a single face. In the latter case, the opposite face is provided with a reflecting coating. The white LEDs may be arranged on a single wall of the plate body, or on two opposite walls of the plate body, or on all the walls that define the peripheral edge of the plate body.
Another subject of the invention is an alternative embodiment, in which the white-LED devices are used to illuminate from outside a panel bearing the image to be displayed, the said panel having diffusing prisms in points corresponding to selected areas of the image, shaped in such a way as to produce a predefined solid angle of light diffusion, the central axis of which is inclined with respect to an axis normal to the panel. This characteristic is exploited for panels designed to be arranged vertically in order to orient downwards the light rays diffused by the diffusing prisms arranged on the panel. This alternative solution is characterized by a higher overall efficiency, which enables a minimal number of LEDs to be used and by the fact that it does not require the use of a costly transparent plate.
In the said embodiment, a lens, preferably operating in total internal reflection and set in front of the LED, enables angular control of the light beam that is incident on the panel. A prism or, alternatively, a reflector, makes it possible to direct the portion of the light beam that diverges in the opposite direction towards the panel.